This invention relates to an improved process for manufacturing lithium batteries, and in particular a process that may be employed in an open environment.
Batteries are comprised of a positive and a negative electrode separated by an ionically conductive, but electrically insulating, electrolyte. Secondary batteries are further defined by reversible electrochemical reactions. Secondary batteries come in a wide variety of types and sizes, but are generally defined by the mobile ion. Thus, lithium secondary batteries typically rely upon conduction of the mobile lithium ion, Li+.
Typically, liquid electrolyte lithium batteries are fabricated under vacuum conditions because both the electrolyte, such as LiPF6, and the metallic lithium negative electrode react violently with moisture in the ambient atmosphere. Solid state lithium batteries are also manufactured under vacuum conditions due to two important factors. First, the most popular electrolyte and electrode materials for solid state batteries also react with moisture. In fact, many thin film batteries use lithium metal as the negative electrode and Lithium cobaltite (LiCoO2) as the positive electrode. Second, solid state batteries depend on an amorphous thin film electrolyte, for which there are few known methods of fabrication.
Previous studies of thin film lithium batteries often focused on the use of lithium phosphorus oxynitride (LiPON) as the electrolyte. The relatively high ionic conductivity and stability in contact with metallic lithium make LiPON a popular choice. Ionic conductivity, however, is heavily dependant on the nitrogen content and thus is limited to vacuum deposition methods.
Two alternate electrolyte materials, lithium metaborate (LiBO2) and lithium sulfide (Li2S) glasses, have also been found to be good lithium ion conductors. Although they provide good conductivity, sulfide glasses tend to be unstable both in contact with lithium metal and under atmospheric conditions. LiBO2 electrolytes have also been found to be unstable with lithium metal but do not typically have similar problems under atmospheric conditions. It was also been found that phosphorous additives, such as P2O5, can further increase the ionic conductivity.
Similar to liquid electrolyte batteries, most solid state lithium batteries utilize metallic lithium as a negative electrode. Metallic lithium is popular because it supplies a high electrochemical potential and thus open circuit voltage (OCV). Although toxic, corrosive and flammable, lithium metal can be manipulated under a controlled environment.
Alternatively, thin film batteries can be developed as an intercalated, or rocking chair, battery. The intercalated battery is a specific type of secondary lithium battery in which both the anode and cathode are formed with intercalation compounds rather than metallic lithium. In this case, the elemental lithium is impregnated, or intercalated, in an oxide rather than applied directly. The lithium ions then move back and forth between interstitial sites as the battery is charged and discharged. While this often reduces the open circuit voltage (OCV) of the cell, intercalated batteries have found niche applications due to improved safety characteristics and power-to-weight ratios.
More recently, lithium impregnated materials have been investigated as potential electrode materials. In 1995 it was shown that the high temperature phase of LiCoO2 shows good stability and reversibility. Oriented vanadium (III) oxide has also been shown to be a potential electrode material.
For solid electrolytes, amorphous thin films are typically preferred because grain boundaries tend to inhibit lithium ion movement within the electrolyte. Because lithium is propagated in solid state ionic conductors by an interstitial method, amorphous or nanocrystalline materials show consistently higher ionic conductivity than do their crystalline counterparts. Unfortunately, only select techniques are capable of depositing thin amorphous films. To this point, the deposition of dense, amorphous, lithium-containing films has often used vacuum or controlled environment processes.
In the last several years, numerous thin film lithium batteries have been developed and commercialized. Thin films are usually considered to be less than 10 microns thick. The Handbook of Thin-Film Deposition Processes and Techniques (Noyes Pubs. 1988; Schuegraf, K. K. editor) provides a broad review of thin-film deposition techniques. These technologies include chemical vapor deposition, pulsed laser deposition, e-beam evaporation and DC/RF sputtering.
Some of the first thin film lithium batteries were developed based on an amorphous lithium phosphosilicate electrolyte. Unfortunately this electrolyte was unstable in contact with metallic lithium and little progress was made until the advent of lithium phosphorus oxynitride (LiPON). LiPON electrolytes were found to be stable up to 5.5V versus lithium metal, which encouraged the development of experimental prototypes. Lithium boride (LiBO2) and lithium sulfide (Li2S) glasses were also found to be good lithium ion conductors. While providing excellent conductivity, sulfide glasses were shown to be unstable both in contact with lithium metal and under atmospheric conditions. In contrast, LiBO2 electrolytes were found to be unstable with lithium metal but did not have similar problems under atmospheric conditions.
A variety of intercalated electrodes were developed to replace lithium metal. In 1995 it was shown that the high temperature phase of LiCoO2 shows good stability and reversibility. More recently oriented Vanadium (III) Oxide was shown to be a potential electrode material.
Solid state intercalated lithium batteries are typically manufactured in a controlled environment using thin film deposition methods such as chemical vapor deposition, pulsed laser deposition, DC/RF magnetron sputtering or e-beam evaporation. Such time and energy intensive methods are required due to the material choices and the difficulty in producing amorphous lithium ion conductors. However, thin film intercalated lithium batteries could be produced much more cheaply and efficiently if thin film, amorphous electrolytes could be developed in the ambient atmosphere.